1. Field of the Invention
The invention relates to a receiver comprising, arranged in this order, an input section, an FM demodulator, to which a frequency-modulated input signal is applied, and an LF section, which FM demodulator comprises a pulse shaper and a low-pass filter. The invention also relates to such an FM demodulator.
Such a receiver is generally applicable for the reception of radio broadcast signals and/or television broadcast signals, in which the sound is transmitted as a frequency modulation of a carrier wave. Such an FM demodulator is suitable for general use, inter alia in the receiver defined above.
2. Description of the Related Art
Funkschau no. 15 of 1970, pp. 500-502, describes an FM demodulator for use in a receiver, which FM demodulator comprises a pulse shaper and a low-pass filter. The pulse shaper is used for generating a pulse of constant shape and width on at least one of the edges of the frequency modulated signal (in the prior-art FM demodulator). This yields a pulse train in which the time between two successive pulses varies with the modulation. The modulation signal is obtained by filtering this pulse train with a low-pass filter. Such a pulse count FM demodulator is substantially linear over a wide frequency range so that when it is used in a receiver, the distortion as a result of demodulation is very low in comparison with that in receivers comprising an FM demodulator of another type. This linearity over a wide frequency range renders this pulse count FM demodulator suitable for use in inter alia a multi-standard television receiver for demodulating the frequency-modulated sound whose carrier frequency may have a large range of values depending on the system standard.
A drawback of a receiver comprising such a pulse count FM demodulator is that the FM component in the output voltage of the FM demodulator is limited by the carrier frequency of the frequency-modulated signal. In radios, for example, the maximum swing is limited to 75 kHz at a carrier frequency which is converted to 10.7 MHz, which means that the modulation component in the output voltage is less than 0.75%. As a result, the demodulator is more sensitive to noise, leading to a deterioration of the signal-noise ratio in the receiver. One possible solution is to reduce the intermediate frequency of the receiver, which improves the ratio between the swing and the intermediate frequency. This requires, for example, additional components such as a mixer and a filter. Another possibility of reducing the influence of noise is to increase the pulse amplitude. However, in the case of implementation in an integrated circuit, the pulse amplitude cannot be increased to a unlimited extent because of the low supply voltage normally used in integrated circuits.